Oxidized activated carbon for the control of pH and alkalinity in water treatment applications

ABSTRACT

An oxidized activated carbon having a contact pH between 7.1 and 8.2 for general use in aqueous treatment systems to prevent pH and alkalinity excursions during start-up of the system and an oxidized activated carbon having a contact pH between 6.0 and 9.5 for specific use in aqueous treatment systems to control pH and alkalinity changes during water treatment.

CROSS REFERENCE TO OTHER PATENT APPLICATIONS

This is a continuation-in-part application of Ser. No. 060,590, filedMay 11, 1993 U.S. Pat. No. 5,368,738.

FIELD OF THE INVENTION

The present invention relates to a process for the control of pH andalkalinity excursions prevalent during the start-up phases of activatedcarbon aqueous adsorption systems. More particularly, this inventionrelates to an oxidized activated carbon for pH and alkalinity control.

BACKGROUND OF THE INVENTION

In the start-up of activated carbon treatment for aqueous systems, it istypical to experience unacceptable increases in the pH and/or alkalinityof the effluent. As used herein, the term "alkalinity" shall mean thewater's acid-neutralizing capacity and is defined as the sum of alltitratable bases. These unacceptable levels of pH or alkalinity contentscan last from several hours to several days. For example, excursionshave been experienced for over 500 bed volumes. This effect has alsobeen found for activated carbons already on-line and subjected to suddenchanges in influent water chemistry (e.g. pH). When these excursionsoccur, the treated water does not meet the standards for distribution tothe customer or discharge to the environment. This problem can lead to asignificant loss in production, environmental problems, or expensiveremedial actions.

The pH/alkalinity excursion phenomenon has been found to occur forvarious types of water treatment applications such as municipal water,industrial process water, ground water, and home water filterapplications. It has been found to exist using various types ofactivated carbons such as those produced from bituminous, subbituminous,wood, coconut, peat feedstocks, or those which are acid-washed prior touse.

The presence of these pH/alkalinity excursions has been a recurringproblem throughout the industry for many years. Notwithstanding theproductivity losses associated with these excursions, little or nothinghas been done to overcome or alleviate the problem. pH/alkalinityexcursions have been largely tolerated because no solution was known toexist. Because of the growing concern for the environmental problemsassociated with these excursions as well as the economic losses, it isan object of the invention to provide a method for preventing oreliminating them. Accordingly, it is an object of the present inventionto provide an oxidized activated carbon that is useful in preventing pHand alkalinity excursions in the start-up phase of aqueous adsorptionsystems.

SUMMARY OF THE INVENTION

Generally, the present invention comprises the use of a mildly oxidizedactivated carbon to prevent the occurrence of pH/alkalinity excursionsduring the purification of water. Various methods of oxidation can beused including the use of oxidizing acids. It is generally preferred toexpose a granular activated carbon to an oxidizing agent, such as air oroxygen, at temperatures greater than 300 degrees C., but less than 700degrees C., preferably between 350 degrees C. and 500 degrees C.

The oxidized activated carbon is characterized by a reduced contact pH¹,in particular a contact pH less than 8.2 but greater than 7.1. Becausethere are no significant changes in other commonly measured activatedcarbon parameters such as hardness, the preferred range is about 8.2 to7.4. In specific cases, pH or alkalinity changes during waterpurification must be minimized rather than providing, for example, agenerally accepted pH range of 6.5 to 8.5. For these cases, the oxidizedactivated carbon is characterized by a reduced contact pH, in particulara contact pH less than 9.5 but greater than 6.0. The oxidized activatedcarbon of the present invention is used in adsorption/filtration systemsfor the purification of aqueous influents. Moreover, the novel oxidizedcarbon can be employed to prevent pH and alkalinity excursions in thestart-up of granular activated carbon (GAC) systems. Other advantages ofthe present invention will become apparent from a perusal of thefollowing description of presently preferred embodiments taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are graphical representations of the pH and alkalinityevolutions of effluents for a Type A GAC versus the number of bedvolumes processed for two types of influents, respectively;

FIG. 3 is a graphical representation of the intensity of the pHexcursion versus the modified contact pH of a given carbon determinedwith a sodium sulfate solution influent (80 mg sulfate/L spiked inultrapure water); and

FIG. 4 is a graphical representation of the evolution of the effluent pHversus the number of bed volumes processed for a Type A carbon beforetreatment and after treatment at two different temperatures using anaqueous system containing sodium sulfate.

FIGS. 5 and 6 are graphical representations of the alkalinity evolutionversus the number of bed volumes processed for a Type A carbon beforetreatment and for oxidized carbon after treatment at two differentcontact pH's.

PRESENTLY PREFERRED EMBODIMENTS

Experimental

Tests were conducted with a one (1) inch inside diameter, one (1) footlong Pyrex glass column containing 60 g of granular activated carbon andusing an empty bed contact time (EBCT) of 7.5 min. The carbon was boiledfor fifteen (15) minutes prior to loading the column. Fines were removedby backwashing with deionized water. Runs were performed in an upflowsystem in order to prevent air build-up in the bed. Each test was runcontinuously for five days and samples of the influent and effluent werecollected over time for further analysis.

Analytical

pH, alkalinity and free carbonic acid concentrations were determinedaccording to standard methods. Anions most commonly found in naturalwaters (sulfate, chloride, nitrate, phosphate, and fluoride) wereanalyzed with a Dionex Model 14 Ion Chromatograph (Dionex Corp.,Sunnyvale, Calif.). The system included a precolumn (AG3, Dionex Corp.),an analytical column (AS3, Dionex Corp.), a micromembrane suppressor(AMMS, Dionex Corp.), a conductivity detector, a 0.024M sodiumcarbonate/ 0,003M sodium bicarbonate eluent, a 0.025N sulfuric acidregenerant and a 50 μL injection loop. Sodium, calcium, and magnesiumwere determined by atomic absorption.

As used herein, the contact pH value of a given activated carbon wasdetermined by adding 25 g of GAC into a 80 mg SO₄ ⁻ /L sodium sulfatesolution prepared in Ultrapure Milli-Q plus water (vide infra). Thisvalue was used as a test for the prediction of pH spikes. The solutionwas gently stirred. The solution pH was determined after a contact timeof thirty (30) minutes. The standard method for determining the contactpH by adding the same amount of carbon to distilled/deionized water (nosodium sulfate added) and a contact time of five (5) minutes was foundnot to be effective in predicting the ability of activated carbonproducts to create pH excursions. Accordingly, the test described abovewas used.

Water Sources

Three water matrices were used for this study: (1) Robinson Township(Pa.) city (tap) water (2) St. Louis (Mo.) city (tap) water and, (3)water prepared from an Ultrapure Milli-Q Plus water system (Milli-Qsystem, Millipore Corp., Bedford, Mass.) having a resistivity of 18.2micromho/cm and a dissolved organic carbon (DOC) less than 0.5 mg/L. Formost of the tests presented here, Milli-Q water was treated with sodiumsulfate to obtain a 80 mg sulfate/L sodium sulfate solution.

Activated Carbons

Tests were performed with a variety of commercially available activatedcarbons, representing most of the major Calgon Carbon and competitiveliquid-phase products. The products tested originated from a wide rangeof raw materials. Adsorption and physical properties of the variousactivated carbons used in these tests are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        Specifications of granular activated carbons (GAC) used                       in the tests.                                                                                    1. Particle size                                           GAC                2. A.D. (g/cc)                                                                            1. Iodine No.                                  type   Raw material                                                                              3. MPD (mm) 2. Ash content (%)                             ______________________________________                                        A      bituminous  1. 12 × 40                                                                          1. 880                                                            2. 0.562    2. 4.91                                                           3. 1.070    (4.2*; 0.3**)                                  B      bituminous, 1. 12 × 40                                                                          1. 1045                                               acid washed 2. 0.492    2. 4.32                                                           3. 1.070                                                   C      coconut     1. 12 × 30                                                                          1. 1245                                                           2. 0.483    2. 2.14                                                           3. 1.090                                                   D      pyrolyzed ion                                                                             1. 20 × 50                                                                          1. 1180                                               exchange resin                                                                            2. 0.513    2. 0.1                                                            3. 0.403                                                   E      subbituminous                                                                             1. 12 × 40                                                                          1. 1050                                                           2. 0.485    2. 8.1                                                            3. 1.050                                                   F      lignite     1. 12 × 40                                                                          1. 625                                                            2. 0.407    2. 17.2                                                           3. 1.190                                                   G      wood        1. 08 × 30                                                                          1. 603                                                            2. 0.266    2. 3.0                                                            3. 1.780                                                   H      bituminous  1. 08 × l6                                                                          1. 768                                                            2. 0.456    2. 12.8                                                           3. 1.430                                                   I      wood        1. 10 × 35                                                                          1. 973                                                            2. 0.212    2. 7.3                                         J      coconut,    1. 16 × 40                                                                          1. 1005                                               acid washed 2. 0.552    2. 0.2                                         K      peat        1. 08 × 30                                                                          1. 859                                                            2. 0.253    2. 5.6                                                            3. 1.120                                                   L      bituminous  1. 08 × 30                                                                          1. 922                                                            2. 0.488    2. 4.9                                                            3. 1.700                                                   M      bituminous  1. 12 × 40                                                                          1. 996                                                            2. 0.509    2. 6.1                                                            3. 1.940                                                   N      bituminous  1. 04 × 10                                                                          1. 1045                                                           2. 0.490    2. 6.0                                                            3. 3.010                                                   O      bituminous, 1. 08 × 40                                                                          1. 878                                                reactivated 2. 0.574    2. 8.3                                                            3. 1.12                                                    ______________________________________                                         *Acid washed with HCl                                                         **Acid washed with HF/HCl                                                

Particle size, A.D. (apparent density), MPD (mean particle diameter),Iodine No., and Ash content were determined according to Calgon Carbontest methods number 8, 7, 9, 4, and 5, respectively.

Carbon Oxidation According to the Invention

Oxidized Type A carbon (Filtrasorb F200 manufactured by Calgon CarbonCorporation) was prepared by treating the carbon with air attemperatures greater than 300 degrees C., but less than 700 degrees C.,preferably between 350 degrees C. and 500 degrees C. The carbon wasoxidized for a time greater than 5 minutes, but less than 3 hours. Acharge of 250 to 1,000 gms of carbon was introduced in a 6 or 12-inchdiameter rotary kiln. The carbon was oxidized with air or oxygen at flowrates between 2.5 and 50 l/min, and at rotation speeds ranging from 1 to10 rpm.

OXIDIZED ACTIVATED CARBON FOR THE CONTROL OF PH AND ALKALINITY IN WATERTREATMENT APPLICATIONS

Effluent pH values were above the Safe Drinking Water Act SecondaryMaximum Contaminant Level (8.5) for more than 350 bed volumes whenvarious types of water (tap and synthetic waters) were processed througha Type A GAC bed typically used for water treatment applications(Filtrasorb 200, Calgon Carbon Corporation, Pittsburgh, Pa.). Alkalinityvalues followed a similar pattern, with effluent values greater thaninfluent values by 5 to 30 mg/l for the same duration. Results areplotted on FIGS. 1 and 2 for two types of water: (1) tap water, and (2)synthetic sodium sulfate solution prepared in ultrapure Milli-Q water(influent pH=6.0). Similar plots were obtained with synthetic solutionsof sodium chloride, sodium nitrate and sodium acetate.

The presence of anions commonly found in natural waters, such assulfate, chloride and nitrate, was found to trigger the effect. Nosignificant pH/alkalinity excursion was reported when anion-free Milli-Qwater was processed through the carbon bed, or when GAC treatment waspreceded by ion exchange treatment.

pH and alkalinity excursions were accompanied by a partial anion removalfrom the solution. Capacities of the activated carbons exhibiting pHspikes for sulfate, chloride, and nitrate ranged from 2 to 9 mg/g GAC,depending on the water characteristics (e.g. pH and presence of otheranionic species), the carbon type, and the nature and concentration ofthe anion. The capacity sequence was found to be SO₄ ⁻⁻ >NO₃ ⁻ >Cl⁻.

For a given carbon, the presence and extent of pH excursions were notsignificantly dependent on the water matrix quality. For instance, theeffect was similar in intensity and duration for tap waterscharacterized by different inorganic contents and for syntheticsolutions of sodium sulfate (80 mg sulfate/L), sodium chloride (18 mgchloride/L) or sodium nitrate (10 mg nitrate/L). There was a thresholdanion concentration (approximately 10 mg/L for most common anionsencountered in natural waters--sulfate, chloride, nitrate) above whichthe effect was no longer concentration dependent. This threshold contentis likely to be present in most natural waters to be treated. Moreover,anion concentrations as low as 1 mg SO₄ ⁻⁻ /L or 5 mg NO₃ ⁻ /L were ableto trigger significant, although less intense, pH spikes.

The ash content of activated carbon did not contribute significantly tothe effect. Significant pH excursions occurred with low ash coconut andacid washed bituminous based carbons. Moreover, a thorough acid-washingprocedure of a bituminous based carbon (leading to an ash content as lowas 0.3% compared to 5.0% initially) did not significantly shorten theeffect.

The ability of a given activated carbon to exhibit pH excursions couldbe predicted by the measurement of its contact pH (using the modifiedtest method). Granular activated carbons with a modified contact pHvalue greater than 8.2 were found to trigger pH and alkalinity spikeswhen placed on-line for water treatment. Most activated carbons testedwere characterized by high contact pH's and exhibited significant pHexcursions. The relationship between modified contact pH and pHexcursion is given in Table 2 and plotted on FIG. 3.

                  TABLE 2                                                         ______________________________________                                        Presence and intensity of pH excursions versus the                            contact pH of activated carbon.                                                                                     Sulfate                                                                       capacity                                                    pH spike  Duration                                                                              (mg/g                                   GAC type Contact pH (Y/N)     (BV)    GAC)                                    ______________________________________                                        A        10.35      Y         350-400 4.5-7.5                                  A*      9.2        Y         300     5.3                                      A**     9.1        Y         250-300 5.2                                     B        9.8        Y         200-250 4.9                                     C        10.35      Y         200-250 3                                       E        10.4       Y         350     6.9                                     G        9.6        Y         550     3.8                                     H        8.6        Y         170     2.6                                     K        11.1       Y         460     4.6                                     D        8.2        N          0      0.7                                     F        7.8        N          0      1.7                                     I        2.1        N          0      0.5                                     J        7.4        N          0      0.5                                     ______________________________________                                         *Acid washed with HCl                                                         **Acid washed with HF/HCl                                                

The presence and intensity of pH excursions were determined with asodium sulfate solution (80 mg sulfate/L) prepared in ultrapure Milli-Qwater.

Activated carbon products that did not exhibit pH excursions did notsignificantly remove sulfate ions from water (Table 2).

Several bituminous based activated carbons (F200 (A), F300 (L), F400(M), BPL (N), Calgon Carbon Corporation), a reactivated product O(Calgon Carbon Corporation) and a coconut based carbon PCB (C) (CalgonCarbon Corporation) were, thus, air-oxidized according to the proceduredescribed above. After treatment, the carbons were analyzed for theirresulting modified contact pH and conventional column tests wereconducted.

                  TABLE 3                                                         ______________________________________                                        Oxidized activated carbons for the conntrol of                                pH/Alkalinity.                                                                           Contact pH before                                                                           Contact pH after                                     GAC type   treatment     treatment                                            ______________________________________                                        A          10.35         7.4 (Tl)                                                                      7.1 (T2)                                             L          11.2          7.7                                                  M          11.1          7.4                                                  C          10.5          7.7                                                  N          10.8          7.4                                                  O          11.8          7.5                                                  ______________________________________                                    

T1 and T2 are two different oxidation temperatures between 300 and 700degrees C.

Contact pH's are presented in Table 3 for all carbons tested, before andafter treatment. The modified contact pHs of these air-oxidized carbonsdropped significantly and were below 8.2, demonstrating a successfultreatment. The pH profiles are presented in FIG. 4 for Type A GAC,before and after treatment at two different temperatures (T1 and T2,respectively). No significant pH or alkalinity rises occurred withoxidized activated carbons. As expected, column tests performed foroxidized F200 showed that the anion exchange capacity of the carbon wasdrastically reduced after oxidation (to 2.5 and 1.0 mg/g for T1 and T2,respectively).

The physical properties of the F200 activated carbon were notsignificantly modified under mild oxidation conditions and theobtainment of a contact pH between 7.6 and 8.2. The particle size,apparent density and ash content were similar before and aftertreatment. Process yields were greater than 98%. Reductions in abrasionnumbers were less than three units. Iodine numbers were reduced by 30 to60 units.

To best control the pH and alkalinity, the contact pH of the treatedcarbon should closely approximate that of the water to be purified. Thealkalinity profile in FIG. 5 shows that the oxidized carbon at pH=7.8controls the alkalinity better for Robinson Township water with a pH of8.5 than does an oxidized carbon at pH=9.0 or the virgin carbon.Further, for a high pH water (City of St. Louis) at 9.2, the oxidizedcarbon at pH=9.0 controls alkalinity better than the virgin carbon oroxidized carbon at pH=7.8.

While presently preferred embodiments of the invention are described inparticularity, the invention may be otherwise embodied within the scopeof the appended claims.

What is claimed is:
 1. In a method for treating water with activated carbon, the improvement comprising controlling pH and alkalinity changes during operation of said water treatment by contacting said water with an oxidized activated carbon having a contact pH below 9.5.
 2. In a method of claim 1 wherein said oxidized carbon has a contact pH between about 6.0 and 9.5. 